1. Technical Field
The present disclosure is related to a magnetic resonance imaging (MRI) apparatus for imaging a subject based on magnetic resonance data with regard to a magnetic resonance in the subject.
2. Description of the Related Art
When a heart is imaged by MRI, image quality deterioration caused by cardiac movement is desired to be suppressed. Especially, it is important that image quality deterioration due to cardiac movement is suppressed in an imaging method requiring high spatial resolution, such as coronary artery imaging or myocardial delayed enhancement. As an imaging method which suits such needs, a method to collect data selectively in a period which has less cardiac pulsation within a cardiac cycle is known by (Stuber, M. et al., “Submillimeter Three-dimensional Coronary MR Angiography with Real-Time Navigator Correction: Comparison of Navigator Locations,” Radiology 1999; 212:579-587). In this method, data is collected during a period determined by a predetermined delay time and data collecting time (window time), which originates from an R-wage obtained from the subject's electrocardiographic (ECG) waveform.
FIG. 5 illustrates an example of a pulse sequence in an MRI method which collects data of a specific cardiac time phase in a cardiac cycle. Data is collected during a period which starts at a starting time point Ts, where the cardiac movement reduces, and ends at an ending time point Te, where the cardiac movement resumes. However, usually, the starting time point Ts is a point of time where a delay time Td has lapsed after the R-wave appears in the subject's ECG waveform. Further, the ending time point Te is a point of time where the window time Tw has lapsed from this starting time point Ts. In this manner, data is collected only in a period with less cardiac movement, which is generally referred to as a ventricular diastole or a slowed inflow phase. The period with less cardiac movement refers to a period which has less change in left ventricle volume (a period in which the chart of the left ventricle volume is flat) as shown in FIG. 5. By collecting data during this period, image degradation caused by cardiac movement can be suppressed.
As shown in FIG. 5, a pre-pulse irradiation is performed during the period until which the delay time Td lapses. For instance, the pre-pulse is an inversion pulse, a T2 weighting preparation pulse, a magnetization transfer contrast (MTC) pulse, a dummy shot, a fat suppression pulse, or a pulse to detect respiratory movement. The inversion pulse is a pulse to improve the contrast of an image in the case of, such as, coronary artery imaging or myocardial delayed enhancement. The T2 weighting preparation pulse is a pulse to T2 weighting. The MTC pulse is a pulse to improve contrast using two or more spin-system magnetization transfer. The dummy shot is a pulse to encourage a nuclear spin to achieve a steady state. The fat suppression pulse is a pulse to suppress a fat signal.
A length of a period of low cardiac pulsation is known to change depending on, for instance, the heart rate of a subject. Accordingly, in order to improve image quality, it is preferred that an appropriate delay time Td and window time Tw are set for each subject. A method to support the setting of an appropriate delay time Td and window time Tw for each subject is proposed in (Plein, S. et al, “Three-Dimensional Coronary MR Angiography Performed with Subject-Specific Cardiac Acquisition Window and Motion-adopted Respiratory Gating,” AJR; 180:505-512, 2003). In this method, an operator can visually determine the period of low cardiac pulsation by, for instance, performing brief cineradiography, which indicates cardiac movement.
The number of collectable data lines N within one heartbeat can be obtained from a repeating time TR of a pulse sequence and the window time Tw in the following equation.N=Tw/TR 
For example, assuming a case in which three-dimensional data is collected when the number of slices, i.e., the number of slice encodes Kz is 60, and the number of matrix Ky in the phase encode direction is 120, the number of required data lines is obtained by the following equation as 7200 lines.Kz×Ky=60×120=7200
When the window time Tw having less cardiac movement within one cardiac cycle is 100 msec, if the repeating time TR is 5 msec, the number of data lines N collectable within one heartbeat is obtained by the following equation as 20 lines.N=100/5=20
The heart rate required for collecting all data lines necessary for image reconstruction can be obtained by the following equation as 360 heartbeats.7200/20=360
When counting one heartbeat as one second, data collection will be completed in 360 seconds, i.e., six minutes. However, in general, other than the cardiac movement, it is also necessary to consider the body movement caused by the subject's respiration. Therefore, in many cases, a method of selectively collecting data which is less influenced by body movement caused by respiration is simultaneously used. In such case, the actual required data collecting time is further extended.
The method proposed in Plein is considered to perform effectively in the case where the heart rate of the subject is significantly stable. However, in some cases, the heart rate of the subject may change during the period in which data collection is performed over a long time as mentioned above. If the heart rate rises, duration of a time phase with less cardiac movement becomes shortened, and an appropriate value of the window time Tw also becomes shorter. For example, suppose the window time Tw as shown in FIG. 6 is set in accordance with an R-R interval Trr1 shown in FIG. 6. When the R-R interval is shortened to Trr2 shown in FIG. 6, the point of time where the cardiac movement increases changes from Te1 to Te2. In this case, the heart will move largely during the period of data collection. Further, in the case of FIG. 6, data collected during period Pa will be significantly influenced by cardiac pulsation and will cause a blur in the reconstructed image.